Am or Am7, generally. The common Dorian vamp would be Am7 - D7.
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Generally, when you're wanting to make up a modal vamp, you want to go between the tonic and the chord that has the "flavor" note of the mode, IE, F Lydian has a #4 compared to the major scale, so you'd want to go between Fmaj7 to G7 for example. As this makes use of the #4 in the Lydian mode.

For the Dorian mode, the flavor note is the major 6th. So I'd say a vamp like Am7 - D7 or Am7 - Bm7.